The present invention relates to semiconductor devices and methods of making same and more particularly to a semiconductor device, suitable for realizing a high multi-computer in which a plurality of processors are connected in high density, and a method for making same.
In order to perform high speed calculations in a computer, it is necessary to mount semiconductor elements in high density on a wiring board, to take as many signals as possible from the respective semiconductor elements and to connect them to each other. One system of taking many signals from semiconductor elements is CCB (Controlled Collapse Bonding). In this system, input/output terminals are disposed in a two-dimensional structure on a semiconductor substrate on which circuits are formed and connected through low melting-point metals such as solder balls. Thus, areas required for taking signals are reduced to thereby achieve high density wiring connections.
Another method, for example, as disclosed in JP-A-62-73651 (laid-open on Apr. 4, 1987), is to provide feed-through paths which electrically connect a front and a back surface of a semiconductor substrate, and to connect fine pins to the feed-through paths on the back surface of the substrate to take signals from the semiconductor substrate. The feed-through paths in this system are formed by a thermomigration technique, as disclosed in PROCEEDINGS OF THE INTERNATIONAL CONFERENCE ON WAFER SCALE INTEGRATION, Jan. 1989, pp.55-64.
However, when semiconductor elements are made finer and the number of input/output terminals on the substrate further increases, these connection terminals are required to be shrunk further. Therefore, distortion under thermal expansion is rapidly increased due to thinning of the connection terminals to thereby rapidly deteriorate the reliability of connection, for example, of the life-time for interconnection under thermal cycling in a system where semiconductor elements and the input/output terminals on the substrate are connected through solder balls, as utilized in the CCB system.
According to the system disclosed in JP-A-62-73651, an increase in the aspect ratio (length/diameter) of the pins reduces distortion under thermal expansion, so that a fine connection structure having a high reliability of connection is achieved compared to the CCB system. According to the present system, input/output terminals are provided on the front and back surfaces of the semiconductor substrate, and wiring conductors are provided which connect the input/output terminals within the substrate. Therefore, laminating the semiconductor substrates in a three-dimensional structure and connecting the input/output terminals achieves a higher degree of integration. Since in the thermomigration for the formation of the feed-through paths, however, small aluminum pieces are disposed on an N-type wafer substrate, a thermal gradient of about 150.degree. C./cm is provided from the front surface of the wafer toward its back surface within an atmosphere of about 1000.degree. C., and thermal diffusion of aluminum into the Si to provide P-type through hole conductors, dimensional accuracies of the individual through hole conductors and the accuracy of electrical conductivity are determined by the accuracy of dimensions and alignment of the aluminum pieces and the accuracy of control of the environmental temperature of the wafer. Therefore, reduction of the pitch of the through hole conductors, an increase in the number of terminals or an increase in the size of the wafer renders difficult alignment and uniform heating of all the through hole conductors for formation of the through hole conductors with high accuracy. Since heat treatment at a high temperature such as about 1000.degree. C. is performed, thermal distortion would remain in the resulting wafer. As the semiconductor elements are narrowed, the characteristics of the devices are greatly deteriorated.
Further, when in the conventional techniques conical holes formed on the wafer back surface are filled with low-melting point metals, the fluidity of the low-melting point metals is reduced as the holes are narrowed to thereby produce voids within the low-melting point metals. Therefore, stress is concentrated on the periphery of the voids to greatly reduce the reliability of the connection. In addition, significant unevenness occurs in the electrical conductivity of the through hole conductors. In elements such as bipolar elements where a large current is consumed, a voltage drop across the power source terminals varies from element to element, so that there is a probability of erroneous operation of the circuits.